X-ray detection



CADMIUM SULPHIDE FIG! April '19, 1955 GO SECS.

GADMIUM SELENIDE 9 x 92: P520123 dd .5213 3530 CADMHUM SELENIDE L 2 a 4 o INVENTOR:- JOHN E. JACOBS BY.

ATT'Y United rates Patent lens X=l AY DETECTIQN John E. Eacohs, dilwaukee, Wis, assignor to General *lectric ompany, a corporation of New York firiginal application May 25, 195i, Serial No. 228,333. Divided and this application July 7, 1954, Serial No. 441,873

ii Claims. (Ci. 25tl83.3)

The present invention relates in general to photosensitive semi-conductors, and has more particular reference to current amplifying semi-conductor materials of unusual photosensitivity particularly well suited for X-ray detection purposes, the same comprising subject matter divided from a copending application for United States Letters Patent, Serial No. 228,333, which was filed May 25, 1951.

Light, broadly speaking, comprises energy emanations or rays of vibratory character having wave lengths within the range of the spectrum, and including visible light rays as well as rays of invisible character, such as ultra violet and infrared rays, X-rays, gamma rays, electromagnetic waves, and other rays of vibratory character including rays comprising alpha and beta particles and electrons, the various ray categories being identified each by its characteristic vibratory frequency or wave length range, commonly expressed in Angstrom units or centimeters.

For the purpose of the present disclosure, a photosensitive semi-conductor may be defined as a material having electrical resistance, or reactance, or both, which vary in accordance with the intensity of rays to which the material is exposed, a particular semi-conductor usually being usefully responsive only to rays within a limited wave length range, characteristic to the semi-conductor, and relatively or entirely unresponsive to other rays.

Electrical resistance and electrical reactance, either in ductive or capacity reactance, or both, are the characteristics of electrical conductors which tend to prevent or impede the how of electrical current therethrough under the influence of an eleotromotive force. The combined current tlow resistive effect of resistance and reactance, in a given conductor material, is commonly referred to as the electrical impedance of the material.

A semi-conductor, in the absence of light rays to which it is responsive, may have impedance characteristics of such high order as to constitute the material substantially as an insulator capable of preventing the flow of electrical current therethrough; or the impedance, while or". high order, may permit minimal current to flow in the absence of exciting rays. The current, if any, thus allowed to flow, in the absence of irradiation may be termed the dark current. When irradiated with rays to which it is responsive, the impedance of a semi-conductor may be reduced as a proportional function of incident ray intensity, whereby the material becomes electrically conductive in proportion to the intensity of exciting rays irnpinging thereon, and consequently will allow proportionally larger current flow in the semi-conductor. The ability of a semi-conductor thus to alter its impedance in response to the intensity of incident light rays impinging thereon, may be employed for many useful purposes, by connecting the semi-conductor in suitable electrical translation systems designated to perform, or to control the performance of, desired work operations, in response to ray induced changes in the impedance of the so connected semiconductor.

Perhaps the most widely known semi-conductors are those which are particularly responsive to visible light, or to invisible rays, such as infrared and ultra violet rays, of wave length adjacent that of visible light in the light spectrum, such semi-conductors being virtual insulators except when exposed to light rays in the visible light spectrum and adjacent infrared and ultra violet spectral regions. These commonly known semi-conductors are not sufliciently responsive to X-rays to allow the useful application thereof to X-ray detecting purposes; in fact, no semi- Lil EfidtifiilZ is, i955 conductor material, effectively responsive to X-rays, was known prior to the invention set forth in a copending application for United States Letters Patent, Serial No. 190,801, filed Gctober 18, 1950, on the invention of John E. Jacobs, relating generally to the novel use of cadmium sulphide as a semi-conductor having current amplifying characteristics for photosensitive purposes, and specifically to the employment of cadmium sulphide as an X-ray detector.

An important object of the present invention resides in providing an eliective X-ray responsive semi-conductor material, which is not only sensitive to X-rays but also to light rays within a wide wavelength band, including visible light rays, the material however being especially well suited for X'ray detecting purposes.

Another important ob ect of the invention is to provide effective X-ray responsive control means adapted to the performance of any desired control function, including X-ray inspection of subjects or objects requiring inspection, X-ray intensity control, regulation of the operating power supplied to X-ray generator equipment, interval timing of X-ray application, and any other operation desirably accomplished in response to the existence of or intensity or duration of detectable X-rays.

Another important object is to apply cadmium selenide as a photosensitive semi-conductor material; a further object being to apply the material as a sensitive X-ray derector, and to provide for employing the same effectively in the detection of X-rays as well as other rays.

Another important object resides in the provision of means for employing cadmium selenide as a semi-conductor and for applying the same eiicctively as a ray detector, by rendering its ray responsive characteristics exceedingly sensitive; a still further object being to provide for sensitizing cadmium selenide as a ray detector by applying thereto, as a sensitizing bias, light rays of selected wavelength different from that of the rays to which it is desired to render the semi-conductor sensitive.

Still another object is to provide for biasing cadmium selenide, specifically for X-ray sensitivity, by applying thereto a visible light bias having wavelength of the orde1 of 5200 Angstroms.

T he foregoing and numerous other important objects, advantages, and inherent functions of the invention will become apparent as the same is more fully understood from the following description, which, taken in connection with the accompanying drawings, discloses a preferred embodiment of the invention.

Referring to the drawings:

Fig. l is a diagrammatic showing of apparatus embodying a semi-conductor for ray detecting purposes;

Figs. 2 and 3 are graphical charts illustrating the response of cadmium sulphide as an X-ray detector; and

Figs. 4 and 5 are graphical charts illustrating the response of cadmium selenide as an X-ray detector, in accordance with the teachings of the present invention.

Briefly stated, the present invention provides for the detection of penetrating rays, such as X-rays, by using crystalline cadmium selenide as a detector, the response of the detector to incident rays'being determined by measuring the alternatin" current impedance of the detector material, as distinguished from its direct current resistance. In accordance with a preferred mode of practicing the invention, the impedance of the detector is measured in terms of electrical potential produced in an impedance measuring circuit connected with the detector, such potential being applied, as through a suitable translation system, to control the actuation of a relay comprising a load device operable in response to predetermined variation in the intensity of penetrating rays impinging upon the detector. The invention also teaches the possibility of increasing the response sensitivity of cadmium selenide as a ray detector by applying light rays having wave length of the order of 5209 Angstrorns as a bias on the detector.

To illustrate the invention, the drawings show a semiconductor element 11, interconnected in a suitable electrical translation system 12, designed to measure the impedance of the element in terms of electrical power delivered to a load device connected with the output side of the system. T he load 13, of course, may comprise any suitable means for the performance of any desired operation in response to changes in the measured impedance of the element 11.

While any suitable or preferred translation system may be employed, the same, as shown in Fig. l of the that ings, preferably comprises an electronic amplifier including an amplifying tube having an anode plate 15, a cathode 1-6, and a control grid 17, the plate 1.5 and cathode 16 being interconnected in an output circuit, including a suitable source of plate circuit power it; and the operable device or load 13. The control grid 17 is int rconnected in a control circuit in which the element 11 is also operatively connected, in order that the grid 17 may be electrically energized in accordance with the transitory impedance values of the element ll.

As shown, the control circuit may comprise the element 11, a preferably uni-directional po er source 1?, and a ballast or control resistor 2d interconnected in serie so that electrical potential corresponding with the impedance characteristics of the element 11 may be developed at the opposite ends 21 and 22 of the resistor The control grid 17 may be connected with the control circuit at the connection point 21, preferably through a condenser 23%, for filtering -dir ponents and allowing the application only of uctuating voltage components on the grid 17. If it be desired to apply unidirectional as well as fluctuating voltage components, on the control grid 17, the condenser 23 may be eliminated; and, if desired, means may be substituted for, excluding fluctuating voltage components while passing only uni-directional voltage components to the grid, if it be desired to control the load device 13 in response to such uni-directional components.

Means for applying a suitable bias between the cathode 16 and grid 17 may also be provided, the same preferably comprising a suitable source 24 of grid biasing power and a regulating resistor 25 interconnected in series with the power source 24- between the cathode and the grid, the connection point 22 of the control circuit being connected with the grid bias means, as at a connection point between the cathode 16 and the resistor 25.

The element 11 comprises photosensitive semi-conductor material, preferably of the sort affording current am plification characteristics in proportion to the intensity of light rays impinging thereon. Cadmium sclenide (CeSe) is such a semi-conductor, as also are mercuric sulphide (HgS) and cadmium sulphide (0313). Semiconductors operate as such through the release of electrons entrapped in the material, such electron release eing accomplished as the result of ray impingement on the material. Commonly known semi-conductors, such as selenium, operate to release electrons in direct proportion to the alteration of the electrical space charge of the substance as the result of exposure of the substance to light rays. Semi-conductors having current amplifying characteristics, however, operate by releasing many thousands or hundreds of thousands of electrons in response to alteration of the space charge. As a consequence, semi-conductors having amplification characteristics, when excited b the impin ement of light rays thereon, operate in fashion comparable to the operation of an electronic amplifying device, whereas selenium and other common semi-conductors do not show such current amplifying characteristics.

The present invention not only takes advantage of the current amplifying characteristics of selected semiconductors, of which ca dmimn selenide, cadmium sulphide and mercury sulphide are examples, but also teaches that the responsiveness of the material may be substantially enhanced by activating the semi-conductor. To this end, the semi-conductor may be activated by applying atomic traces, of the order of one part in one million, of foreign matter, in the molecular lattice structure of the material.

Crystals of cadmium sulphide, mercury sulphide and cadmium selenide may be produced by vapor phase chemistry procedures, wherein vaporized constituent substances are mixed, under controlled conditions, and from which mingled vapors crystals of the desired material may be deposited. Cadmium or mercury sulphide crystals, for example, may be, deposited from the mingled vapors of cadmium or mercury and su l urated hydrogen. Cadmium selenide crys .ls, likewise, may be deposited from the mingled vapors of cadmium and selenium. Mingled vapors may be heated in a retort at temperatures of the order of 900 C. and may then be delivered to a relatively cooler chamber or zone in which crystals of the desired material may grow on the walls of the chamber at temperatures of the order of 600 C. and below. The resulting crystals may have atomic impurities, such as sulphur, selenium and the like, distributed more or less at random in the lattice structures thereof, which tend to impart photosensitive qualities in the material, such qualities being absent in material that is entirely free of lattice impurities. Atomic particles of metals, such as manganese, copper, indium and silver, may also be applied in the crystaltine lattice structure for the purpose of activating crystalline cadmium or mercury sulphide, or cadmium selenide, for use as ray sensitive semi-conductors, no claim being made herein for crystalline material so activated, nor to the method of applying atomic particles of metal in the lattice structure of crystalline material since the same forms subject matter of invention claimed in the aforesaid copending application for U. S. Letters Patent Serial No. 228,333, filed May 25, 1951.

When a crystal ll of the sort herein contemplated is exposed to X- ays emanating as from a ray source 26, the impedance of the crystal changes substantially in proportion to the intensity of impinging X-rays. Where the applied rays are of pulsating character, the impedance change in the crystal follows the pulsations of the impinging rays and consequently establishes a corresponding pulsating voltage across the resistor 2%, which, being applied to the control grid 17, produces corresponding power pulsations for application to the load device 13. Irradiation of a crystal with X-rays of fluctuating intensity results in the development, across the resistor 2% of voltage having a uni-directional as well as a fluctuating component.

X-rays produced by operation of the usual X-ray generating tubes, electrically excited by alternating current power at 60 cycles, comprise X-ray energy pulsations at a frequency corresponding with that of the energizing power applied for the operation of the ray generator. X-rays of uniform, non-pulsating character may, of course, be produced and applied upon the crystal 11, in which case the voltage developed across the resistor 29 will be of unidirectional character. Consequenty, the translation systern 12 should be designed to measure the magnitude either of the uni-directional impedance of the crystal or the fluctuating impedance thereof, depending upon the unidirectional or fluctuating character of the impinging rays.

Semi-conductors of the character herein contemplated exhibit impedance chances when exposed to visible light rays. as from, a light source 2'7, the extent of such visible light induced impedance change is proportion to the intensity of rays impinging on the crystal from the source 27. Accordingly, when a crystal is exposed to rays to which it is sensitive, from a source other than the X- ray source 26, the voltage available at the connection points 21 and 22 may contain COllT'Oilfii'riS which corre spond with the impedance value of the crystal, determined by the light rays from. such other source as well as components corresponding with the rays emanating from the X-ray source as. if the rays impinging on the crystal from the source 27 are of uniform intensity, the

corresponding voltage component across the resistor Zi w'dl be uniform. Where the impinging X-rays are of fluctuating character, the same may be applied through the condenser 23. for the control of the amplifier l t, while the uniform voltage component established by illumination of the crystal n the source 27, at uniform intensit as well as the uni-d rectional X-ray induced voltage component, will be .luded from the amplifier system by the action of the cor tnser The present invention, of course, is not limited to the excitation of the cryst l or other rays from a source 27 and by pulsating X-rays from th. source 2.6, or in its broadest aspects, the invenap s to the excitation of the crystal 2 by means or Visit light or by means of X-rays, or both, and whether or not the light rays or the X-rays Julsate, there being many possible advant ous applications involy. the excitation of the cr stal e er by X-rays or by other li'ht rays including visible light, where either the light rays or X-rays are of uniform or of pulsating intensity character.

of -n in connection ystal prises grooves green light, in the visible light spectrum and having a wave length of the order of 5200 Angstrom units; by applying such a light bias, it has been discovered that the crystal is rendered highly sensitive to impedance changes in response to X-ray irradiation, X-ray induced impedance fluctuations, when the crystal is under light bias at a wave length of 5209 Angstroms, being of the order of ten times the X-ray induced impedance fluctuations produced in the crystal by X-ray excitation thereof in the absence of a light bias.

The detection characteristics of the crystals do not alter upon exposure thereof to X-rays and other ut rays. In this connection, the performance of hexagonal cadmium sulphide and cadmium selenide crystals, in eluding standard crystals containing usual lattice impurities, and crystals activated with selected metals, have been investigated, using X-rays, having wave length of 1.54 Angstroms, for crystal excitation. The X-ray beam thus applied to the examined Crystals was of pulsating character at a frequency of 60 cycles. To determine its impedance response characteristics, a suitable unidirectional elect'ornotive force, of the sort supplied by the source it? in Fig. 1, may be applied to the crystal and the resultant current fiow therethrough accurately measured. The resulting crystal current obtained in response to pulsating X-ray irradiation of the crystal was found to contain an alternating as well as a uni-directional cornponen. The uni-directional component was found to vary substantially linearly with the intensity of incident X-rays, the alternating component substantially as the square of incident X-ray intensity. This phenomenon is explainable upon the theory that the alternating component is proportional to the rate of recombination of electrons in the effective conduction band or Zone of the irradiated crystal.

The magnitude of the uni-directional component of crystal current in crystalline cadmium sulphide and cadmium seleuide was found to be approximately 10,000 times that of the alternating component at intensities of approximately quanta/second, in standard cadmium and mercury sulphide crystals containing natural impurities as activating media in the crystal structure, as well as in crystals activated with selected metallic media. The ratio was found to be somewhat variable, each crystal having its own characteristic ratio. in cadmium selenide crystals the ratio was often found to be somewhat lower than that for cadmium sulphide, indeed cadmium selenide crystals have been examined in which the ratio w as low as till At higher incident intensities this to is reduced, and it supposed that, with increased ray intensity, the ratio approaches unity.

In standard CdS crystals, the time lag before crystal current reaches a maximum value following initial application of the X-ray beam is of tie order of several minutes so far as the uni-directional crystal current component is concerned, but is of the order of /6 of a second for the alternating component, in the same crystal, as shown in Figs. 2 and 3. Cadmium selenide crystals, however, respond to X-rays several times faster than do CdS. As compared with CdS, the time lags of the unidirectional and alternating components of crystal current in standard cadmium selenide crystals, containing natural impurities as activating media, are respectively of the order of one minute and or the order of second, as shown in Figs. 4- and 5.

The curve 28 in Fig. 2 shows the approximate alternating current impedance response obtained from standard CdS crystals, when irradiated with X-rays of intensity lioh fluctuatin between maximum and minimum at a fre-,

quency or cycles per second. The curve 28 in Fig. 4 shows the corresponding response obtained under similar conditions for a CdSe crystal. The curves 29 and 29 in Figs. 3 and 5 respectively illustrate the direct current resistance characteristics of standard crystalline cadmium sulphide and cadmium sele ide, containing natural impurities as activating media, as measured in terms of unidirectional current tlow in the crystalline material following X-ray application. The curves 2% and 29 show the direct current response as Well as the alternating current response of cadmium selenide to be substantially faster than that of cadmium sulphide, as depicted by the curves 28 and 29. The curves 35) and 3% respectively illustrate the decline of the uni-directional crystal current component from a maximum value following discontinuation of X-ray application upon cadmium sulphide and cadmium 5 selenide crystals. Upon termination of X-ray irradiation on CdS and CdSe crystals, the llow of the alternating crystal current component ceases immediately upon the conclusion of the cycle of crystal current flow then in being.

it will be noted that, in cadmium selenide as well as cadmium sulphide crystals the time lag required for the uni-directional current component to reach a maximum value is many times that within which the fluctuating current component of the crystal reaches its maximum value. This phenomenon may be explained upon the theory that the iluctt :1 component is a measure of the change in the number of electrons present in the conduction band of the crystal, as a function of time. Accordingly, employment of the fluctuating component of crystal current, to the exclusion of the uni-directional component, will permit the almost instantaneous measurement of crystal current for the determination of X-ray intensit'-, thus avoiding the mor; extended delay necessary to achieve a stable condition in using the uni-directional current oonent. Measurement of the fluctuating current cc conent only permits the advantageous use of high gai; fluctuating current ampli'lers in the translation system, in the interests of effective instrumentation.

Some crystals are found to exhibit random or eccentric impcdance characteristics in the absence of irradiation, at polarizing field strengths in excess of volts per centimeter. Other crystals, while not exhibiting such random characteristics, in the absence of irradiation may show erratic effects when irradiated. Crystals havingthese erratic characteristics are those containing structural imperfections and lattice distortions, as evidenced by asterism in Laue transmission patterns of the crystals. Crystals which are free from lattice distortions show none of these erratic or eccentric characteristics.

Employment of the alternating component of crystal current, for -ray detection purposes, permits the application of secondary illumination as a light bias to the crystal for increasing the X-ray sensitivity thereof. When such a light bias is directed on a crystal, sensitive to the bias irradiation, while simultaneously irradiated with pulsating X-rays, both the uni-directional and fluctuating components of X-ray induced crystal current are increased by a factor of the order of 10, as compared with such X-ray induced components in the absence of the light bias. Each diiferent kind of crystal may have a corresponding optimum light bias wave length producing maximum response, the response being reduced by variation of the light bias wave len th above or below the optimum Wave length value characteristic of the crystal. For cadmium sulphide crystals, however, regardless of the manner of activation, the optimum sensitizing eifect is obtained in response to light bias having a wave length of the order of 5200 Angstroms.

In. all crystals that have been examined, currents in excess of 10 times the current resulting from primary ionization of the crystal in response to X-ray irradiation, were observed. Any explanation of this phenomenon must account for the release of additional electrons in the crystal to produce the observed current multiplication. The energy necessary to produce this additional or amplified crystal current can only be derived in the crystal itself. Accordingly, the crystals comprise excess electron or donor type semi-conductors which operate, in a sense, as current amplifiers under the control of visible, as well as invisible, light rays impinging thereon. In this connection, it is thought that electron donor centers in the crystals are ionized by impingement thereon of primary electrons, equilibrium occurring when electrons are being trapped to form the donor centers at the rare at which such centers are being ionized.

The present invention primarily visualizes the practical application of crystals for improved instrumentation in association with X-ray generators and auxiliary equip ment. The invention additionally contemplates the possibility of using the crystals for any light detecting purposes to which the same may be suited. The crystals may be employed separately for detection purposes by placing the crystal in the path of the ray beam to be detected. So positioned, the crystal, in association with appropriate translation equipment of the sort shown in Fig. l, or other modified translation systems for application of the principles herein revealed to particular purposes, may be used for many control purposes, including, or example, the control of the intensity of the irradiating beam at a desired value, by applying the load device 13 to control suitable equipment for directly or indirectly regulating the intensity of the beam. The apparatus may be employed for liquid level gauging purposes. A crystal and its associated translation system may be employed as a timing device to discontinue the application of the ray beam after a selected time interval, which may be determined either in terms of time or in terms of ray quanta. Many other uses to which the invention be applied will suggest themselves to those skilled in the electronic art.

Several crystals, each with its associated translation system, may be mechanically arranged to form a sensitive screen for the examination of obiects. Such screen may comprise a multiplicity of crystals mounted with their relatively negative ends facing toward the ray source to be detected, equipment. embodying such screens bei especially useful in the examination oi: products, eluding packaged food products, f flaws, impurities, or other characterzst or" the o'niect under examination. Since the crystals can be made in relatively small sizes, it is obvious that a detection s reen of line grain, comprising a, multiplicity of closely arr "aged crystals, can be made for the detection of exceedingly small features of the examined object, such as small impurities in packaged food and other products.

It is thought that the invention and its nume tendan'r advantages will be fully understood Loom the foregoing description, and it is obvious that numerous changes may be made in the form, constructio i id arrangement of the several parts without depin from the spirit or scope of the invention, or sacrix a any of its attendant advantages, the for herein d closed being a preferred embodiment for the urpose of illustrating the invention. No claim is herein to either cadmium or mercury sulphide for penetrating ray detecting purposes since the same respectively form the subiect matter of invention claimed in copending applications for U. 3, Letters Patent Serial No. 190,801, tiled Gctober 18, 1950, on the invention of John E. Jacobs, relating generally to the use of cadmium sulphide as a penetrating ray detector, and Serial No. 232,073, filed lune l8, 195i, on the invention of John a Jacobs and Rudolf Frerichs, relating generally to the use of mercury sulphide as a penetrating ray detector.

The invention is hereby claimed as follows:

i. The method of detecting changes in the intensity level of pulsating X-rays, which comprises the application of said pulsating rays upon cadmium selenide, as a semi-conductor, to thereby rapidly alter the alternating current impedance of the semi-conductor, as a p function of the intensity of impinging X-ra s, while simul taneously changing the direct current resistance thereof at a relatively slow rate, and measuring the altern current impedance of the semi-conductor as distinguished from the direct current resistance thereof.

2.. The method set forth in claim 1 including t plication of visible light rays of selected wave lea. a sensitizing bias on the semi-conductor.

3. The method set forth in claim 1, including the piication of visible light rays having wave length of the order of 52-38- Angstrorns as a sensitizing bias on the set-nu conductor.

4. The method of detecting changes in the intensity level of pulsating X-rays, which comprises the ap lication of said pulsating rays upon cadmium selenide, as a semi-conductor, to thereby rapidly alter the alternating current impedance of the semi-conductor, as a precise function of the intensity of impinging X-rays, white simultaneously changing the direct current resistance thereof at a relatively slow rate, producing a flow of current in h e semi-conductor proportional to the instantaneous values of its direct current resistance and alternating current impedance, isolating the alternating current component of said flow of current from the direct cl. nt component thereof, and actuating an operable dev :e in response to change in X-ray intensity level as measured by said alternating current component.

5. Control apparatus for actuating an operable device in response to rapid change in tr e intensity level of pulsating Y-rays comprising cadmium seienidc a crystalline semi-conductor element having alternating current impedance characteristics, variable precisely and substantially instantly as a function of the intensity of pulsating X-rays impinging thereon, and direct current load resistance characteristics which laggingly follow any change in pulsating ray intensity, means for continuously passing a flow of current in said element, means for isoiating the laggingly responsive direct current component of said current from the alternating current component thereof, and electrical translation means controlled in accordance with said alternating current component for operating the load device substantially instantly in response to rapid changes in the intensity level of said X-rays.

6. Control apparatus as set forth in claim 5, including means to apply on said semi-conductor element a light comprising visible light rays having a selected wave length.

7. Control apparatus as set forth in claim 5, including means to apply on said semi-conductor element a light bias comprising visible light rays having wave length of the order of 5209 Angstroms.

8. Control apparatus for actuating an operable load device in response to rapid change in the intensity level of pulsating X-rays comprising cadmium selenide as a crystalline semi-conductor element having alternating current impedance characteristics, variable precisely and substantially instantly as a function of the intensity of pulsating X-rays impinging thereon, and direct current resistance characteristics which laggingly follow any change in pulsating ray intensity, a resistor connected in s res with said element, means for continuously passing a flow of current through said resistor and element to develop potential fluctuating as a function of the laggingly responsive direct current resistance and the precisely responsive alternating current impedance of said element, an electronic amplifier having a control grid and drivingly connected with said load device for actuating the same, and a coupling condenser for applying on said grid a controlling potential corresponding with the instantaneous values of the alternating current impedanc characteristics of said element.

9. Control apparatus for actuating an operable load device in response to rapid change in the intensity level of pulsating X-rays comprising a cadmium selenide crystaliine semi-conductor element having alternating current impedance characteristics, variable precisely and substantially instantly as a function of the intensity of pulsating X-rays impinging thereon, and direct current resistance characteristics which laggingly follow any change in pulsating ray intensity, a measuring circuit in series connection with said element for circulating thcrethrough a flow of electric current having alternating current and direct current components respectively proportional to the instantaneous values of the impedance and resistance characteristics of said element, an electronic amplifier having a control grid and drivingly connected with said lead device for actuating the same, and a coupling network interconnected with said circuit and said grid for applying said alternating current component on said grid while excluding said direct current component t .ierefrom.

1!). X-ray detection apparatus comprising a load device to be operated in response to the detection of X- rays, crystalline cadmium selenide forming a detection element adapted for exposure to X-rays, means for applylight rays of Wave length of the order of 5260 Angiroms and of selected substantially constant intensity, as a sensitizing bias on said detection element, means to measure the impedance of said element comprising an electrical power source and a resistor connected in circuit with said detection element, an electron flow amplitier having a control grid, said ampln'ier being controllingly connected with said load device, and a coupling condenser interconnecting the grid of said amplifier with said circuit to actuate the amplifier in accordance with the alternating current impedance of the detection elenent as measured in said circuit.

l1. X-ray detection apparatus comprising a load device to be operated in response to the detection of X-rays, crystalline cadmium selenide forming a detection ele ment adapted for exposure to the action of X-rays, means for applying light rays of selected wave length and or". selected substantially constant intensity as a sensit ng bias on said detection element, means to measure. impedance of said element comprising an electrical power source and a resistor connected in circuit with said de'ection element, an electron flow translation device having a control grid, said translation device being controllingly connected with said load device, and a coupling con- O denser interconnecting the grid of said translation device OTHER REFERENCES with said circuit to actuate the device in accordance with the anemafingcurrent impedance of the daection e1 Crystal Counters-HofstadterNucleon1cs April 1949,

. pp. 2-27. ment as measured m sald Guam 5 On the Conductivity Produced in CdS Crystals by Irradiation With Gamma-Rays-Frerichs Phys. Rev. vol. References Clted 1n the file of thls patent 7 #2, Dec. 15, 19 PP- 1869-1875- UNITED STATES PATENTS Photoelectricity-Allen 1913. Publ. by Longrnans,

Green & Co., New York, N. Y., pp. 75-79. gfgg 2? 10 Physical Rev. Series II, vol. II, 1917, pp. 305-306. 2505633 Whale A 5 56 The Physics of Electronic SemiconductorsPears0n 2508098 chilogslz g 1950 Technical Paper 47-34 published by Bell Telephone 2537388 wooldridge 1951. Laboratories Inc., New York, December 1946, pages 3 323 zg g w Photo-Conductivity of Incomplete Phosphors-Fre- 2,546,239 Rothsghi-laf 1951 richs Phys. Rev., vol. 72, #7, Oct. 1, 1947, pp. 594-601. 2,547,173 Rittner Apr. 3, 1951 2,604,596 Ahearn July 22, 1952 

1. THE METHOD OF DETECTING CHANGES IN THE INTENSITY LEVEL OF PULSATING X-RAYS, WHICH COMPRISES THE APPLICATION OF SAID PULSATING RAYS UPON CADMIUM SELENIDE, AS A SEMI-CONDUCTOR, TO THEREBY RAPIDLY ALTER THE ALTERNATING CURRENT IMPEDANCE OF THE SEMI-CONDUCTOR, AS A PRECISE FUNCTION OF THE INTENSITY FO IMPINGING X-RAYS, WHILE SIMULTANEOUSLY CHANGING THE DIRECT CURRENT RESISTANCE THEREOF AT A RELATIVELY SLOW RATE, AND MEASURING THE ALTERNATING CURRENT IMPEDANCE OF THE SEMI-CONDUCTOR AS DISTINGUISHED FROM THE DIRECT CURRENT RESISTANCE THEREOF. 